There is known a sheet processing apparatus for executing a specified post-processing on a sheet (paper) conveyed from an image forming apparatus. The sheet processing apparatus includes a processing section for executing a post-processing and a standby section arranged above the processing section. The standby section temporarily retains subsequent sheets while the processing section executes the post-processing on the sheet. If the processing section is idle, the standby section drops the retained sheet towards the processing section. The sheet falling down to the processing section is subjected to a post-processing such as a sorting processing and a stapling processing. After the post-processing, an extrusion member of the processing section operates to extrude an edge of an upstream side in a conveyance direction of a sheet towards the downstream side. In this way, the sheet subjected to the post-processing is conveyed to the downstream side of the conveyance direction to be discharged from the processing section.